Suprathermal electrons are a major heat source of ionospheric plasma. How the suprathermal electrons evolve during their bounces inside the plasmasphere is a fundamental question for the magnetosphere‐ionosphere coupling. On the basis of Van Allen Probes observations and quasi‐linear simulations, we present here the first quantitative study on the evolution of suprathermal electrons under the competition between Landau heating by whistler mode hiss waves and Coulomb collisional cooling by background plasma inside a plasmaspheric plume. We show that the Landau heating can prevail over the collisional cooling for >50 eV electrons and cause the field‐aligned suprathermal electron fluxes to increase by up to 1 order of magnitude within 1.5 hr. Our results imply that the plasmaspheric plume hiss waves could mediate energy from the ring current electrons to the ionospheric plasma. Plain Language Summary The ionospheric plasma temperature variation can affect the Earth's atmospheric escape and the spacecraft orbit decay. A major heat source of the ionospheric plasma is the suprathermal (several eV to hundreds of eV) electrons, which can bounce along the magnetic field lines inside the plasmasphere. How these suprathermal electrons evolve during their bounces is a fundamental question for the magnetosphere‐ionosphere coupling. In the past, the plasmaspheric suprathermal electrons were usually considered to gain energy from the ring current ions through Coulomb collisions and wave‐particle interactions. Here we show that strong whistler mode hiss waves can grow from the instability of the ring current electrons in the plasmaspheric drainage plume, and their Landau heating can prevail over the collisional cooling by background plasma for >50 eV electrons. The enhanced field‐aligned suprathermal electrons could eventually heat the ionospheric plasma. These results have significant implications for understanding the energy transfer process from the magnetosphere to the ionosphere. Key Points The collisional cooling by background plasma counteracts largely the Landau heating by hiss waves for <50 eV electrons The Landau heating can increase the field‐aligned fluxes of >50 eV electrons by 1 order of magnitude within 1.5 hr Plasmaspheric plume hiss could mediate energy from the ring current electrons through the suprathermal electrons to the ionospheric plasma